1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly, to a metal alloy for pads (electrodes) and wires (or bumps) of the semiconductor device.
2. Description of the Related Art
Generally, a semiconductor device is constructed by a lead frame, a semiconductor chip mounted on an island of the lead frame, and wires connected between electrodes of the semiconductor chip and leads of the lead frame. The semiconductor device is sealed by a resin molded package.
In a prior art semiconductor device, the electrodes are made of aluminum (Al) or aluminum silicon (AlSi), and the wires are made of gold (Au). Since Au has a large malleability and ductility, the Au wires are hardly broken by the stress of resin while injecting and cooling (or hardening) it. Also, since Au is most inert among metals, the Au wires are hardly eroded by impurities and water included in the resin, and also, the Au wires are hardly oxidized by the atmospheric air.
On the other hand, in a multi-media field or the like, since semiconductor devices have been highly-integrated and operate at a high speed, the heat dissipation per semiconductor chip has been remarkably increased. As a result, the junction temperature T.sub.j of such semiconductor devices needs to be higher than 125.degree. C. Note that the junction temperature of conventional semiconductor devices is lower than 100.degree. C., and even the junction temperature of semiconductor devices specially for automobiles is lower than 125.degree. C. In addition, in order to reduce the junction temperature, the semiconductor devices may be sealed in a ceramic package or a metal package which has a good heat dissipating effect, or may be sealed in a resin molded package associated with a heat sink. Note that any of the ceramic package, the metal package and the resin molded package associated with a built-in heat sink surely increase the manufacturing cost.
Therefore, even in a multi-media field or the like requiring a large power dissipation per semiconductor chip, a resin molded package without such a built-in heat sink is indispensible, in view of the manufacturing cost. In this case, it is required to guarantee a junction temperature of higher than 125.degree. C.
In the above-described prior art resin molded semiconductor device without a heat sink, however, in order to guarantee a junction temperature higher than 125.degree. C., when an acceleration reliability test as a high temperature storage test is applied to the semiconductor device at a temperature higher than 150.degree. C., such as 175 to 200.degree. C., compound layers called "purple plague" or the like may grow between the electrodes (Al or AlSi) and the wires (Au), which deteriorates the combining (bonding) strength therebetween. At worst, Kirkendall voids may be created between the electrodes and the wires, so that the wires are disconnected from the electrodes. This will be explained later in detail.
In addition, the failure rate of resin molded semiconductor devices due to the deterioration of the combining (bonding) strength between the electrodes and the wires is dependent upon the glass transition temperature T.sub.g of resin. That is, the higher the glass transition temperature T.sub.g of resin, the lower the failure rate of semiconductor devices. This will also be explained later in detail.
Note that JP-A-2-119148 discloses that a wire is composed of 1 to 5 wt. percent copper (Cu) with the residual amount being substantially composed of Au. Also, JP-A-4-229631 discloses that a wire is composed of 5 to 30 wt. percent Cu with the residual amount being substantially composed of Au. Further, JP-A-56-49535 discloses that a wire is composed of 1 to 40 wt. percent palladium (Pd) with the residual amount is substantially composed of Au. Thus, the combining (bonding) strength between the electrodes and the wires is improved. According to any of JP-A-2-119148, JP-A-4-229631 and JP-A-56-49535, however, the improvement of the combining (bonding) strength between the electrodes and the wires is insufficient, and no consideration is given to the effect of compound of Cu and Pd.